This invention relates to an improved method and system to acoustically detect the initial touch of a milling tool to a workpiece without false touch indications on extraneous machine noises.
The tool touch detection logic of a Machine Tool Monitor disclosed in copending application Ser. No. 749,845, filed June 28, 1985, now U.S. Pat. No. 4,704,693, C.E. Thomas, was developed for lathe applications and has some capability for milling applications. However, the portion of the logic included to avoid false alarms on noise spikes, sometimes found in machine tool operation noise, extracts a larger penalty in milling operations than in lathe operations. This penalty is in the form of delays between the initial touch event and generation of the touch detection signal, and in penetration of the tool into the part before the detection signal is generated. In order to gain the signal information necessary for the pattern recognition logic to discriminate between the touch vibration signal and spurious noise spikes, the preferred embodiment of this standard logic uses a minimum of six signal samples gathered at the rate of one signal sample every half millisecond. The detection can occur in this minimum time only if each signal sample amplitude is above the detection threshold, and also above the amplitude of the immediately preceding sample. Thus, under these ideal conditions the tool is in continuous contact with the workpiece for a period of only three milliseconds before a tool touch detection signal is generated, and the depth of penetration of the tool into the workpiece continuously increases over this period at a rate determined by the tool feedrate. At the usual touch detection feedrate of one inch per minute, the tool penetrates into the workpiece a negligible 0.048 mil in the three milliseconds between initial tool contact and tool touch detection generation.
The situation for milling tool touch detection is quite different. Individual tool inserts around the periphery of a rotating tool do the cutting, and the workpiece may not rotate. Each tool insert contacts the workpiece and penetrates deeper into it along a circular arc, reaching maximum penetration when the radius from the tool center through the insert is perpendicular to the workpiece surface (see FIG. 4), and continuing to follow the circular arc with reducing depth of penetration until the cutter emerges from the workpiece. The resulting vibration signal is a pulse whose duration equals the total time of cutter-workpiece contact. For the very light cuts most compatible with the tool touch detection concept, these pulses are difficult to distinguish from the spiky noise pulses the current standard tool touch detection algorithm is designed to ignore. Analysis shows that this touch detection logic is least satisfactory for milling tool touch detection when the tool speed is high.
Other relevant commonly assigned patents on tool touch detection are U.S. Pat. No. 4,428,055--Kelley et al; No. 4,554,495 --Davis; and allowed application Ser. No. 645,203--Thomas, et al., now U.S. Pat. No. 4,704,693, U.S. Pat. No. 4,631,683 is an improvement over the last.